(1) Field of the Invention
The present invention relates to an image forming apparatus, and in particular to a technique to reduce deterioration of a photoreceptor due to light-induced fatigue while also maintaining an image quality by neutralizing and erasing electric charge remaining on the photoreceptor by emitting erase light prior to charging.
(2) Description of the Related Art
Image forming apparatuses using an electrophotographic system charge a surface of a photoreceptor to a uniform electric potential with use of a charger, forms a latent image by exposure scanning using a laser beam, develops the latent image using toner, and transfers the developed toner image onto a recording sheet.
However, when the toner image formed on the photoreceptor is transferred, the following phenomenon occurs: a transfer current concentrates at a non-exposure portion (dark portion) where toner does not exist, and electric charge remains at a portion where toner exists (light portion). It is difficult to uniformly charge the surface of the photoreceptor with electric charge remaining thereon partially. As a result, a previous image may appear, as hysteresis (in this specification, referred to as “photoreceptor memory”), in the latent image and the toner image that results from development of the latent image, in the subsequent image forming process. This leads to deterioration in the image quality.
Accordingly, a conventional method of irradiating the photoreceptor with a predetermined light (in this specification, referred to as “erase light”) after the toner image is transferred, to neutralize the potential on the photoreceptor, and subsequently charging the photoreceptor for the next image formation is widely adopted.
The surface of photoreceptors suffers abrasion due to such as a cleaning operation, and the photosensitive layer suffers fatigue, deterioration, and the like. Thus, the lifetime of photoreceptors is limited to a particular period of time, and exposure by erase light is considered to be a factor affecting the lifetime of the photoreceptors.
Specifically, in recent years, Organic Photoconductors (OPC) using organic photosensitive materials are widely used. Although such organic photoreceptors (photoconductors) have advantages in, for example, cost, productivity, and low-pollution characteristics, they are susceptible to light-induced fatigue due to emission of the erase light. The surface of the photoreceptor with light-induced fatigue readily wears down due to a cleaning operation and the like.
Here, as disclosed in, for example, Japanese Laid-Open Patent Application Publication No. 2003-76076 and Japanese Laid-Open Patent Application Publication No. 2005-208223, output of the erase light may be suppressed to decrease image deterioration due to the photoreceptor memory so as to extend the lifetime of the photoreceptor.
However, the degree of the photoreceptor memory is considered to be affected by various factors including a usage environment. Particularly, in image forming apparatuses that have multiple photoreceptors and form a color image by transferring toner images formed on the photoreceptors by multiple transfer, the degree of the photoreceptor memory is considered to be affected by the positions of the photoreceptors.